Speaker
Description
Thermal decomposition of ammonia to hydrogen has received special attention due to the potential use of ammonia as a carbon-free hydrogen storage medium. Supported Ru catalysts are among the most active catalysts showing a NH$_{3}$ conversion of 41 % at 550°C at a WHSV of 30.000 mlg$_{cat}$$^{-1}$h$^{-1}$. Despite their high activity, the scarcity and high cost of Ru limit the applicability of such catalysts. Transition metal catalysts containing Ni, Co and Fe are considered as scalable alternative.
In this project, oxidic host structures containing Ni, Fe and Co are systematically synthesised. The aim is to investigate the formation of an active catalyst for ammonia decomposition depending on the microstructure of the oxidic precursor, which is determined by the synthesis method. Precipitation, (supercritical) hydrothermal synthesis and flame spray pyrolysis were selected as synthesis techniques. Host structures include solid solutions with rock salt structure, spinels and perovskites. A standardised test procedure for catalyst screening is crucial for the comparative investigation of different catalyst materials. Therefore, a handbook protocol was developed by all cooperation partners, which includes the provision of benchmark catalysts for data alignment.
The synthesis of spinel precursors (MgM$_{2}$O$_{4}$, M=Fe, Co, Ni) from hydrotalcite compounds was started with the precipitation of MgFe$_{2}$O$_{4}$ to follow up a recently reported reference.$^{1}$ Phase-pure MgFe$_{2}$O$_{4}$ and a phase mixture of MgFe$_{2}$O$_{4}$ with 9 wt% Fe$_{2}$O$_{3}$ show high NH$_{3}$ conversions of 67 and 73 %, respectively, at 600°C, both higher than 65 % NH$_{3}$ conversion over the reference wüstite-based (FeO$_{x}$) industrial ammonia synthesis catalyst. TEM and STEM analyses of the spent catalysts transferred without air contact revealed the formation of metallic Fe particles in the size range of 20-50 nm containing nitrogen highly dispersed on spherical/needle-like MgO particles.
In the synthesis of nickel-containing catalysts, Ni-MgO solid solutions containing 1, 5 and 10 at% Ni were prepared by co-precipitation and Ni-Al$_{2}$O$_{3}$ systems were synthesized via layered double hydroxide precursors. The nickel metal exfoliated from the solid solution with MgO shows an intrinsically twice as high activity as the Al$_{2}$O$_{3}$ supported Ni and an industrial Ni/Al$_{2}$O$_{3}$ standard, with the apparent turnover frequency increasing with decreasing nickel content. Electron microscopy analysis shows well dispersed Ni particles characterized by a narrow size distribution with the maximum at 4-5 nm in and on the porous MgO matrix. No sintered Ni particles are observed after catalysis. These results are consistent with the high stability of these catalysts. Thus, these Ni-MgO materials represent a good basis for further systematic investigations of structure-function relationships on a system that is definitely also relevant for a technical application.
References
1. K. F. Ortega, D. Rein, C. Lüttmann, J. Heese, F. Özcan, M. Heidelmann, J. Folke, K. Kähler, R. Schlögl, M. Behrens, ChemCatChem, 9, 659 (2017).
| Abstract Number (department-wise) | AC 1.2 |
|---|---|
| Department | AC (Schlögl) |
